A transfer press in which a plurality of pressing stations are provided in series in a press machine to permit a workpiece to be sequentially pressed in these stations, commonly includes a transfer feeder that is adapted to be driven to convey and transfer the workpiece sequentially in the successive press stations.
Such a transfer feeder typically includes a pair of transfer bars arranged parallel to each other and extending in a workpiece feed direction, and a drive means for driving the transfer bars two- or three-dimensionally to transfer the workpiece or to move it in a desired pattern of movement with the transfer bars driven. The known drive means for driving the transfer bars may be either of cam drive type or of servo drive type and can only be operated in either a cam drive mode or a servo drive mode.
For example, Japanese Examined Patent Application No. Sho 62-26848 discloses a transfer feeder that adopts a drive means of cam drive type in which a cam rotated by a power taken out of a press machine causes a lever to oscillate, thereby to cause the transfer bars to be driven, three-dimensionally, in a feed, a lifting and a clamping direction, respectively, with a feed, a lifting and a clamping lever that are interlocked with the transfer bars. The system described has a construction in which the press machine and the transfer feeder are mechanically synchronized in operation, which provides an advantage that an emergency or unexpected shutdown of the press machine if required or if it happens may not cause a movable part of the press machine and a movable part of the transfer feeder to interfere with each other.
In a transfer feeder of cam drive type, however, a transfer parameter such as a lifting stroke or a clamping stroke must be determined by a profile of a cam. Then, if such a stroke needs to be altered, the cam must be replaced. If a plurality of the feed cams need to be used as described in the publication referred to above, those cams must be replaced. Replacing a cam is a time-consuming task in a transfer press system and reduces its productivity. Also, inconveniently enough the requirement to replace a plurality of cams makes the drive system complicated and increases its cost of manufacture.
On the other hand, Japanese Unexamined Patent Publication No. Hei 6-106271 and No. Hei 7-47497 disclose a transfer feeder of servo drive type in which the drive means comprises as its drive source servo motors to drive the transfer bars two- or three-dimensionally. The transfer feeder of servo drive type advantageously permits its feed stroke, lifting stroke and clamping stroke to be readily altered simply with the servo motors controlled, thus of advantage does not require a particular setup operation in order to meet a change in workpiece sizes and shapes as required.
The construction in which the press machine and the transfer feeder are electrically synchronized in operation, however, entails the possibility that development of any abnormality in a path of transmission of signals between them may well injure their synchronization, thus causing a movable part in one of them and a movable part in the other to interfere with each other.
To prevent an interference between a movable part in a press machine and a movable part in a transfer feeder, a measure is described in Japanese Unexamined Patent Publication No. Hei 6-106271, in which when a press machine comes to be stopped by a power failure, the kinetic energy of its upper die descending is converted to an electric energy which is then used to move the die to a safety site where it may not interfere with any movable part in the transfer feeder.
In the arrangement described in Japanese Unexamined Patent Publication No. Hei 7-47497, an angle of rotation that needs to be effected to cause the transfer feeder to halt after the press machine is stopped is preliminarily stored in a memory as a data which is used on an actual occasion as a control data to operate the transfer feeder to prevent an interference between a movable part in the press machine and an movable part in the transfer feeder.
The measure and arrangement adopted in these latter publications are, however, only operative responsive to a trouble indicative signal that can be derived from a path of communication between a press machine and a transfer feeder, and are not effectively applicable to a situation in which an abnormality takes place in a control means for the transfer feeder, causing it to run away. Of inconvenience, then, an interference of the type described cannot, in effect be prevented.